Spring 2015 BE 191 In-Class Activity Describing the Rheological Properties of Unknown Materials

Spring 2015 BE 191 Instructions, Part 1 Assemble into teams of 4 or 5 at a dry erase board or table. Work as a team to feel, pour, squish, squeeze, ..., and deform the four materials (A-D) provided to your group. As a team, identify whether each of your samples is a pseudoplastic/shear-thinning (n <1), Newtonian (n = 1), or dilatant/shear-thickening (n>1) fluid. Also, try to guess what each fluid is! Their smell might give the away. t = shear stress (Pa) ty = shear yield stress (Pa) K = flow consistency index, units of Pa•sn g = shear rate (s-1) n = flow behavior index: n < 1 for a pseudoplastic fluid n = 1 for a Newtonian fluid n > 1 for a dilatant fluid

Spring 2015 BE 191 Answers, Part 1 Corn syrup (Sample A) is a viscous, Newtonian fluid. Its viscosity (~3.6 Pa*s) does not change with shear rate. Corn starch (Sample B) is shear-thickening. Its apparent viscosity is increasing with shear rate. Icing (Sample C) is a shear-thinning, or pseudoplastic, fluid. Its apparent viscosity is decreasing with shear rate. Tooth paste (Sample D) is similar to icing; it’s shear thinning. t = shear stress (Pa) ty = shear yield stress (Pa) K = flow consistency index, units of Pa•sn g = shear rate (s-1) n = flow behavior index: n < 1 for a pseudoplastic fluid n = 1 for a Newtonian fluid n > 1 for a dilatant fluid

Spring 2015 BE 191 Instructions, Part 2 Next, your team should prepare a figure containing estimated shear stress vs. shear rate curves for each of the materials you were given. One of the materials in your possession is similar to a Bingham plastic in that it has a yield stress. Clearly indicate this fluid’s yield stress in your plot. HINT: Your plot will not look like the one to the right! Really think critically about how each of your fluid samples flows. How viscous is it? When does it flow? How easily does it flow? t = shear stress (Pa) ty = shear yield stress (Pa) K = flow consistency index, units of Pa•sn g = shear rate (s-1) n = flow behavior index: n < 1 for a pseudoplastic fluid n = 1 for a Newtonian fluid n > 1 for a dilatant fluid

Spring 2015 BE 191 Answers, Part 2 Corn syrup is a viscous Newtonian fluid. Its flow curve should be a straight line. Corn starch is shear-thickening. At increasing shear rates, this suspension behaves more like a solid, only deforming under ever-increasing shear stresses. At most shear rates, icing has a higher apparent viscosity than the tooth paste. The tooth paste has a yield stress of ~15-30 Pa; when applied shear stress is below this yield stress, it does not flow. Corn starch Icing Tooth Paste Corn Syrup t = shear stress (Pa) ty = shear yield stress (Pa) K = flow consistency index, units of Pa•sn g = shear rate (s-1) n = flow behavior index: n < 1 for a pseudoplastic fluid n = 1 for a Newtonian fluid n > 1 for a dilatant fluid